Glow plug wiring has different ohm ratings (getting bad), glow plug ground flaky or glow plug relay.

  You can use a multimeter to test.  I may be able to assist offline.

Jim Felder <felder@KNOLOGY.NET> wrote:
  Last weeks flurry of posts over this seemingly insignificant issue
was a great help, but I haven't solved the problem. I do have another
data point though. My glow plug LED (it's a diesel) works normally as
it should. However, it will come on very infrequently while I am
driving. It may stay on for some time, or it may be on for a fraction
of a second. If there is a pattern, it's that the light comes on when
I decelerate. The deceleration shifting from fourth to fifth will do
it, so will backing off the throttle at highway speed, so will coming
to a stop in my driveway.

Is there a known source of this loss of ground, or should I just
start looking for a loose wire? I've visually inspected the wiring
mess under the instrument cluster many times, and have never seen
anything that I think looks suspicious.

Thanks,

Jim

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